Acronyms used herein are listed below following the detailed description. Rel-13 LTE LAA (Licensed Assisted Access) provides licensed-assisted access to unlicensed spectrum while coexisting with other technologies and fulfilling various regulatory requirements. As used herein, licensed refers to licensed radio spectrum such as for example conventional cellular band and unlicensed refers to license-exempt radio spectrum such as for example that used by conventional wireless local area networking utilizing various of the IEEE 802.11 radio protocols. In Rel-13 LAA, unlicensed spectrum is utilized to improve the LTE DL throughput. In this solution, one or more LAA DL SCells may be configured to a UE as part of DL CA configuration, while the PCell needs to be on licensed spectrum.
Document RP-152272 by Ericsson and Huawei entitled New Work item on enhanced LAA for LTE [3GPP TSG RAN Meeting #70; Sitges, Spain; 7-10 Dec. 2015] introduces unlicensed band UL operation, and includes some discussion on SRS operation in LAA.
Relatedly, the MulteFire Alliance is developing specifications for MulteFire technology which is to be a stand-alone unlicensed band operation (unlike LTE LAA which requires the assisting PCell to be in the licensed band) in which one requirement is that the MulteFire UL supports SRS. Generally the Multefire Alliance is proceeding by using certain building blocks from LTE LAA, and it is intending to also use building blocks from Rel. 14 eLAA, as much as may be appropriate in order to speed up the development of this LTE technology-based stand-alone operation in the unlicensed bands.
UL sounding reference signal transmissions are an integral part of the LTE system operation. SRS in LTE is used for UL link adaptation (including spatial and multiple-input-multiple-output (MIMO) link adaptation) as well as for UL sounding based DL precoding and link adaptation in the case of LTE TDD/FS2 (Frame Structure 2) utilizing channel reciprocity. SRS operation for unlicensed bands is being developed to have similar use cases.
UL SRSs are sent by the UE in certain specified locations of the radio frame. In relevant part for LTE, that is SRS with the trigger type 1, the UE learns when it should transmit an SRS jointly from the higher layer configured parameters and PDCCH which carries the UE's scheduling allocation. More specifically, the UE learns when it should transmit SRS from its grant of UL resources that it receives in the PDCCH combined with higher layer configuration of periodicity and timing offset of SRS opportunities in relation to the subframe number. But for unlicensed band operation in LTE LAA and in MulteFire a single PDCCH may grant to a given UE multiple UL subframes. To the inventors' knowledge, to date neither LTE LAA nor MulteFire has dealt with the issue of SRS transmissions where a single resource grant gives allocation for multiple UL subframes.
Typically it is not useful for the network to receive an SRS in each of those multiple granted UL subframes since generally the channel conditions across those at least some of those multiple UL subframes would be coherent and a single SRS could adequately represent the channel across all those channel-coherent subframes. If the UE were to always send an SRS in each of those multiple coherent UL subframes all but one of those SRSs would generally represent wasted signaling. At the same time a multiple UL grant in a single PDCCH is generally given to high-traffic UEs, so abstaining completely from receiving SRSs from those particular UEs in those scheduling events would tend to reduce throughput efficiency (by not having precise channel soundings) on the busiest connections. What is needed in the art is a way for the network to instruct or otherwise control the UE to send SRS in only selected ones (if any) of the multiple UL subframes granted in a single PDCCH, and to do so in an efficient way that does not add too much to the control signaling overhead on the unlicensed band. As will be described below, the solution herein for triggering the UE to send SRS works quite well also for triggering the UE to send aperiodic channel state information (CSI) reports which give the network information on quality of the UE's downlink channel, or block ACK/NACK.